1. Field of the Invention
The present invention relates to semiconductor wafer or substrate handling systems and in particular relates to substrate transport spatulas.
2. Description of the Related Art
Integrated circuits which are typically comprised of many semiconductor devices, such as transistors, diodes, and resistors, are often fabricated oil a thin slice of semiconductor material, otherwise known as a wafer. These semiconductor devices are often formed in the wafer in a process that involves raising the temperature of the wafer, depositing layers of material on the wafer, exposing the wafer to doping atoms, and removing portions of material from the wafer so as to selectively form the wafer into a preferred configuration. Consequently, when forming such integrated circuits, it is often necessary to remove the wafer from one processing chamber and reposition the wafer into another processing chamber, while the wafer is at a high temperature state. However, since the wafer is extremely brittle and sensitive to external forces, great care must be taken so as to avoid physically damaging the wafer while it is being transported.
To avoid damaging the wafer during the transport process, wafer transport spatulas are often used. In particular, such spatulas support the wafer by contacting a lower surface of the wafer with an upper surface of the spatula. Furthermore, frictional forces acting therebetween enable the spatula to move the wafer so that the wafer can be repositioned in a manner that prevents the wafer from moving with respect to the spatula.
One type of spatula has a flat upper surface to support the wafer so that the upward force that supports the wafer is distributed somewhat uniformly over the surface of the wafer contacted by the spatula. Thus, the upward pressure that is exerted onto the lower surface of the wafer by the flat spatula is relatively small. Consequently, the reduced pressure applied by the flat spatula onto the wafer inhibits the flat spatula from forming scratches along the lower surface of the wafer while the wafer is supported by the spatula and while the wafer is accelerated by the spatula.
However, flat spatulas have several problems. In particular, entrapped pockets of hot gas are sometimes formed between the flat lower surface of the wafer and the flat upper surface of the spatula, thereby compromising the frictional engagement between the wafer and the spatula. In particular, it is possible for a small enclosed space to be formed between the wafer and the spatula due to either the upper surface of the spatula or the lower surface of the wafer having a slightly concave shape. Furthermore, if the wafer is in a high temperature state, a gas which is confined within the small space will likely heat up and attempt to expand in volume. Consequently, the expanding heated gas will exert an upward force so as to reduce the degree of contact between the upper surface of the spatula and the wafer. Thus, it is possible that the wafer will slide along the upper surface of the spatula while the spatula is moved, which can lead to scratching of the lower surface of the wafer or can even result in the wafer being dropped during movement.
Another problem with flat spatulas is that it is sometimes difficult to remove the wafer from the spatula. In particular, when an external force is applied onto the wafer so as to raise the wafer off of the spatula, a narrow evacuation space is initially created between the wafer and the spatula. However, since the lower surface of the wafer and the upper surface of the spatula are substantially flat and since the resulting distance between the wafer and the spatula is relatively small, surrounding air may not readily flow into the narrow space between the wafer and the spatula. As a result, a region of low air pressure can also sometimes be produced along the lower surface of the wafer. Hence, the air pressure along the upper surface of the wafer can sometimes be much larger than the air pressure along the lower surface of the wafer, thereby requiring an upward lifting force that is substantially greater than the weight of the wafer to remove the wafer from the spatula. It is possible that the required lifting force will damage the wafer or will cause inefficiencies in the manufacturing process.
Another type of spatula known in the art, otherwise referred to as the minimally contacting spatula in this application, has been developed to address the problems of the flat spatula. In particular, the minimally contacting spatula comprises several pins that upwardly extend above the spatula. Furthermore, the minimally contacting spatula engages the wafer by contacting the pins with the lower surface of the wafer so that each of the pins supports the wafer on an equal basis. Consequently, it is important that the pins be precisely aligned in a vertical manner so as to securely engage the wafer with the minimally contacting spatula.
Typically, the minimally contacting spatula employs only three pins. Three is the minimum number of point forces that are required to Support the wafer in a state of static equilibrium, and it is difficult to vertically align more than three pins. Since the flat lower surface of the wafer is not in contact with a broad external flat surface, entrapped pockets of gas are inhibited from forming along the lower surface of the wafer. When lifting the wafer off of the three pins, surrounding gas can readily flow underneath the wafer so as to enable the wafer to be lifted with a force that is only marginally greater than the weight of the wafer.
However, minimally contacting spatulas have several problems. In particular, since the minimally contacting spatula typically contacts the wafer at only three small regions of the lower surface of the wafer, the pressure exerted on the lower surface of the wafer is relatively large at the three regions of contact. Thus, it is possible that the increased pressure exerted on the lower surface of the wafer will damage the lower surface of the wafer. Furthermore, to limit the damage caused by the minimally contacting spatula, it is often necessary to move the wafer at a relatively low rate.
From the foregoing, it will therefore be appreciated that there is a need for an improved wafer transport spatula that operates more effectively than known flat spatulas and known minimally contacting spatulas. In particular, there is a need for an improved spatula that provides sufficient frictional engagement of the wafer so as to enable the wafer to be moved at a more desirable rate and is capable of supporting the wafer in a manner that does not substantially damage the lower surface of the wafer.